Diesel engine vehicles include, in their exhaust gas passages, filters (diesel particulate filters: DPFs) that collect particulates (particulate matter) in exhaust gas, and some of the DPFs are loaded with catalysts. The catalysts are used to promote combustion for removing particulates in order to reuse filters when the amount of particulates accumulated on the filters increases.
Regarding such a catalyzed particulate filter, PATENT DOCUMENT 1 employs a catalyst in which a activated alumina particle material loaded with Pt, a CeZr-based composite oxide (mixed oxide) particle material, and a ZrNd-based composite oxide particle material are mixed together in order to enhance particulate combustibility. PATENT DOCUMENT 2 describes that primary particles of activated alumina, primary particles of Ce-based composite oxide, and primary particles of Zr-based composite oxide are mixed together to form secondary particles in a support, and this support is loaded with catalytic metal in order to promote combustion of particulates. PATENT DOCUMENT 3 shows that Rh and Pt are provided in the crystal lattice or between atoms of composite oxide particles including Ce and Zr and part of Pt is exposed at the surface of the particles in order to lower the combustion temperature of particulates and the temperature at the start of combustion and to prevent sintering of a catalyst precious metal.